In recent years, batteries as power sources of mobile devices and the like have been strongly required to have light weight designs and thin designs. Therefore, also for covering materials for batteries, those using metal thin films or laminated films obtained by laminating a metal thin film and a thermally-fusible resin film have come to be used in place of conventional metal cans having limits in terms of light weight and thin designs. Laminated films are capable of adopting lighter weight designs, thinner designs and more free shapes than metal cans.
As an example of a laminated film used in a covering material for a battery, there is a type in which a thermally-fusible resin film which is a heat sealed layer is laminated on one surface of an aluminum thin film, which is a metal thin film, and a protective film is laminated on the other surface. The laminated film encloses a battery element composed of a positive electrode, a negative electrode, an electrolyte and the like so that the thermally-fusible resin film is disposed on the inner side and thermally fuses the circumference of the battery element, thereby hermetically sealing (hereinafter simply called sealing) the battery element. For example, a polyethylene film and a polypropylene film are used as the thermally-fusible resin film, and for example, a nylon film and a polyethylene terephthalete film are used as the protective film. A lead terminal is connected to each of the positive electrode and negative electrode of the battery element, and these lead terminals are caused to extend to the outside of the covering material.
If a voltage outside a standard range is applied to a battery during the use of the battery, there may sometimes occur the case where gas is generated by the electrolysis of an electrolyte solvent and the inner pressure of the battery rises. Furthermore, if a battery is used at high temperatures outside a standard range, substances which become a source of gas due to decomposition and the like of the electrolyte may be produced.
The generation of gas within a battery brings about a rise in the internal pressure of the battery. To suppress a rise in internal pressure, many batteries using a metal can as the covering material have a pressure safety valve which lets the gas escape to the outside when the inner pressure of the battery has risen. Also in a film-covered battery using a film as covering material, the provision of a pressure safety valve has been studied. For example, JP2000-100399A discloses a film-covered battery in which a portion at high thermal fusion temperature and a portion at low thermal fusion temperature are provided in a sealing region of the film and the portion at low thermal fusion temperature is caused to function as a safety valve. JP11-97070A discloses a film-covered battery which is such that a non-thermally-fusible resin film is interposed in part of a thermally-fused portion. In both cases, a portion where the thermal fusion strength is weakened is formed in the covering material and this portion is given the function of a safety valve.
JP2002-56835A discloses a configuration in which part of the outer shape of a thermally-fused portion is cut off from the outside. JP10-55792A discloses a configuration in which an unbonded portion is provided in part of a sealed side and the dimension of this unbonded portion is increased on the inner side of a battery and decreased on the outer side thereof. In these configurations, a portion having a shape which is such that the width of the thermally-fused portion in the direction from the inner side of the battery to the outer side thereof becomes narrow, is set in part of the thermally-fused portion, and this portion is caused to function as a safety valve.
On the other hand, it is also very important to take insulation measures for a film-covered battery.
FIG. 1 shows a schematic plan view of an example of a film-covered battery based on the technique related to the present invention in which a laminated film is used. FIGS. 2A and 2B show partial sectional views in Line e-e of FIG. 1. FIG. 2A shows the condition before the adhering of a peripheral protective tape and FIG. 2B shows the condition after the adhering of a peripheral protective tape.
Laminated film 100 is a three-layer laminated film which is such that a thermally-fusible resin film, which is thermally-fusible resin layer 104, is laminated on one surface of an aluminum thin film, which is metal layer 103, and protective film 102 is laminated on the other surface.
Film-covered battery 110 shown in FIG. 1 uses two laminated films 100, which are upper laminate 100a and lower laminate 100b. Two laminated films 100 are such that thermally-fusible resin layers 104 are caused to face each other and in sealing position 108 on the circumference of battery element 120, upper laminate 100a and lower laminate 100b are thermally fused, whereby battery element 120 is sealed.
However, if this state is kept as is, as shown in FIG. 2A, metal layer 103 becomes exposed from each section of upper laminate 100a and lower laminate 100b (in the figure, exposed portion 103a of the metal layer). If an assembled battery is fabricated in this state, the risk that exposed portion 103a of the metal layer will come into contact with an adjacent film-covered battery or will come into contact with a surrounding conductive member, increases. Therefore, insulation measures have been taken by covering this exposed portion 103a of the metal layer with peripheral protective tape 101, as shown in FIG. 2B.